This invention relates generally to chemical-mechanical planarization (CMP) and, in particular, to the pad conditioning aspect of CMP.
Chemical-mechanical planarization (CMP) is a well known process used in semiconductor fabrication to polish a surface, for instance a dielectric film surface formed on a silicon wafer or other workpiece. The CMP process removes small elevated features on the surface without significantly thinning the film on, for instance, the flat areas lower than the elevated features.
As shown in a cross-sectional front view (FIG. 1), a typical CMP system uses a flat, rotating disk (xe2x80x9cplatenxe2x80x9d 112) with a pliable polishing pad 110 mounted on its upper surface 112S. As platen 112 is rotated, a slurry (not shown) is deposited near the center of the pad""s surface 110S and spread outward using, at least in part, centrifugal force caused by the rotation. A wafer 106 (or substrate), held by a carrier 102 positioned above pad 110, is then pressed downward against pad""s surface 110S such that the rotating polishing pad 110 moves the slurry over the wafer""s surface 106S. In this manner, elevated spots of the wafer surface 106S are removed and a substantially planar surface is achieved. It is to be noted that although pad surface 110S is illustrated as a smooth surface for simplicity of illustration, in fact pad 110 has a rather roughly textured surface 110S to be rubbed against wafer surface 106S along with the acidic (or basic) slurry containing abrasives.
Pad 110 is made of, e.g., polyurethane impregnated felt, cast and sliced polyurethane with filler, cast and sliced polyurethane without filler, or composite of two or more types of pad material. For effective polishing, the pad""s surface 110S needs to have a flat or bell-shaped profile. After a certain period of CMP polishing, the profile of the pad""s surface 110S is altered to be no longer useful for polishing, resulting in inefficient and sometimes even ineffective polishing. Deterioration of the profile happens as the pad surface loses its rough texture through mesa formation. Therefore, to maintain the quality of planarization, pad""s surface 110S needs to be rejuvenated (i.e., a desirable profile of surface 110S restored) once in a while by, for example, raising the naps of surface 110S by scraping surface 110S.
One problem with prior art pad rejuvenation is that it has to be done xe2x80x9coff-linexe2x80x9d and ex situ, i.e., either before or after the polishing event with the wafer removed from the polishing station, reducing the throughput of the polishing process. Another problem with prior art pad rejuvenation (which is performed ex situ) is that a uniform removal rate is not maintained, since areas of pad surface 110S are conditioned only in the beginning of the polishing process and, as areas of pad surface 110S are used one or more times for polishing, the removal rates of those areas gradually drop.
In accordance with the present invention, an apparatus and method for rejuvenating a polishing pad in situ without interrupting the CMP (or other types of) polishing are disclosed. Embodiments of the present invention achieve uniform removal rate and a higher throughput relative to prior art pad conditioning which cannot be performed during the CMP polishing.
In one embodiment, the inventive apparatus includes a wafer carrier carrying a wafer facing down a platen on which a polishing pad is placed. Along the bottom perimeter of the carrier are a number of conditioning tips, which are retractable upward in the direction normal to the upper surface of the platen by, say for example, spring loaded mechanism known to a person skilled in the art. During the polishing of the wafer surface (which faces the pad on the platen), the wafer carrier rotates in one direction and the pad on the platen rotates in the same or other direction. When the removal rate of the polishing process drops below a permissible value (i.e., the surface of the pad needs conditioning), the retractable tips attached to the rotating wafer carrier are lowered onto the pad to start conditioning the pad. The conditioning tips are pushed against the pad by air pressure so that the tips, while in constant contact with the pad, flexibly conform to the varying topography of the pad surface.
When the surface of the pad is sufficiently conditioned, the retractable tips are raised from the pad to stop conditioning. Later, whenever the state of the pad surface deteriorates after a period of polishing, the process of lowering the tips to condition the pad and raising the tips when the pad surface is conditioned is repeated.